The clotting of blood involves a serial cascade of enzyme, cofactor, proteolytic and gelation reactions mediated by a group of cellular and plasma proteins known as coagulation factors. Initiation of this cascade can occur when the cellular receptor known as tissue factor (TF) binds coagulation factor VII or its derivative, factor VIIa, to form a catalytically active complex. In the absence of TF and without continued binding in a complex, factor VII/VIIa does not initiate coagulation. Thus, the chemical and biological characterization of TF is clearly important to understanding the mechanism of coagulation.
Tissue factor is a membrane-bound glycoprotein that is not normally found soluble in the circulation or accessible to plasma proteins including factor VII/VIIa and the other coagulation factors. While tissue factor is not normally expressed on the surface of cells that form the vasculature, its expression by monocytes within the vasculature can be induced by infectious agent constituents such as bacterial lipopolysaccharide, lymphokines derived from some antigen-stimulated T helper cells, directly by some stimulated T helper cells, and immune complexes. Certain inflammatory mediators of monocyte/macrophage origin, e.g. interlukin 1 and tumor necrosis factor alpha as well as bacterial lipopolysaccharide, can stimulate endothelial cells that line the humoral surface of blood vessels to express TF. Expression of TF in the vascular compartment typically results in disseminated intravascular coagulation or localized initiation of clotting, i.e., thrombogenesis.
Tissue factor is constitutively expressed on the surface of some extravascular cells in in vitro culture including fibroblasts, some as yet unidentified types of brain cells, and certain epithelia that are separated from the circulating plasma proteins by basement membrane barriers. The presence of TF on these cells results in clot formation upon contact with blood as a result of tissue damage. Thus, TF is the foundation upon which the hemostatic system is initiated.
The report of Howell, Am. J. Physiol, 31:1 (1912) was the first to suggest that an isolated tissue protein preparation containing TF could promote coagulation only when present as a phospholipid-protein (lipoprotein) complex. Reconstituting the functional procoagulant activity of TF by relipidating the isolated protein has been necessary because isolation of the TF-containing tissue protein typically results in removal of the phospholipids which are normally associated with the TF protein, such reconstitution has been studied by a number of investigators. For instance, recovery of coagulant activity has been reported to be influenced by the phospholipid type, the ratio of phospholipid to protein, and the detergent and ionic composition of the reconstitution mixture. See Nemerson, J. Clin. Invest., 47-72 (1968); Nemerson, J. Clin. Invest., 48-322 (1969); and Carson et al., Science, 208:307 (1980).
Both isolated and relipidated TF-containing protein preparations have been prepared by extraction from the tissues of various species. Historically, the methods used were difficult, time consuming and resulted in low yields because tissue factor is only present in extremely small quantities in naturally occurring tissues. For a review of the classical methods, see Nemerson et al., Prog. Hem. Thromb., 6:237-261 (1982).
More recently, Broze et al., J. Biol. Chem., 260:10917-20 (1985), Bom et al., Thromb. Res., 42:635-643 (1986) and Guha et al., Proc. Natl. Acad. Sci, USA, 83:299-302 (1986) have reported isolating human tissue factor (huTF) protein using a method based on the discovery that delipidated tissue factor protein can bind factor VII/VIIa when the protein is solubilized in an aqueous solution containing a nonionic detergent and CaCl.sub.2. However, the utility of that method, which employs a factor VII/VIIa affinity sorbent, as a means for isolating tissue factor protein is limited not only by the difficulty in obtaining significant quantities of isolated factor VII/VIIa but also by the lability of factor VII/VIIa.
Broze et al., supra, have suggested that the development of monoclonal antibodies specific for huTF and their use as immunoaffinity sorbents could circumvent problems caused by the limited availability of factor VII/VIIa. However, no anti-huTF monoclonal antibodies have been reported in the literature. Furthermore, two monoclonal antibodies raised against bovine TF [Carson et al., Blood, 662-156 (1985)] do not immunoreact with huTF (Guha et al., supra).